There is an increasing demand for wind turbines delivering an increasing power level. This may lead to an increase in the size and the weight of the wind turbines as well. This, however, constitutes a problem in terms of construction and maintenance of the wind turbines. It is, therefore, desirable to construct larger turbines, i.e., with an increased power level, without thereby increasing the dimensions of the turbine correspondingly.
One way of obtaining this is disclosed in WO 02/14690 disclosing a drive assembly for a wind turbine comprising a rotor hub and a planetary type gear transmission unit comprising sun, planet and ring gears, and a planet carrier. The ring gear is non-rotatably secured to a supporting structure, for example, a turbine nacelle, and a main bearing rotatably supports the rotor hub and planet carrier relative to the ring gear and supporting structure. The sun, planet and ring gears may lie in a transverse plane (perpendicular to the rotation axis of the rotational forces) which also contains the main bearing. Thereby, a compact drive assembly is provided.
It is a disadvantage of this drive assembly that it is relatively difficult and costly to manufacture the drive assembly, partly because the manufacture of the ring gear, being adapted to mesh with the planet gears on an inner surface, is relatively difficult and costly to manufacture. Furthermore, the existing production techniques for such ring gears put a restrain on the possible choice of materials in such a manner that it will normally be necessary to choose more expensive materials. Another disadvantage is that the transverse loads acting on the main bearing deflect the ring gear, and thereby directly disturbs the gear contact. Furthermore, deflections of the supporting structure of the drive assembly also deflect the ring gear, which is solidly bolted to the supporting structure. This also disturbs the gear contact.
Another example of a prior art drive assembly is described in WO 2004/015267, disclosing a wind turbine gear unit comprising a low speed gear module and a plurality of high speed gear modules. The low speed gear module is operable simultaneously to transmit torque to each of the high speed gear modules. The low speed gear module may be either a bull gear or a ring wheel. When using a ring wheel as the low speed gear element the main bearing of the rotor may be situated on the outer diameter of the ring wheel, thereby providing a compact gear unit as described in WO 02/14690. However, this is not possible if a bull gear is used as the low speed gear element, and it is therefore necessary to balance the need for a compact gear unit against the need for a gear unit which is easy and cost effective to manufacture. This is a disadvantage.
Yet another example of a conventional planet gear system is disclosed in WO 02/14690 or DE 103 18 94. In the gear system disclosed in these references each of the planet gears has two tooth meshes, one towards the ring gear and one towards the sun. The tooth forces in these two contacts have opposite direction. This causes alternating bending loads on the planet gear with every revolution. This is a disadvantage because it reduces the capacity of the planet gears.